title: Analyses of cerebral microdialysis in patients with traumatic brain injury: relations to intracranial pressure, cerebral perfusion pressure and catheter placement.
abstract: BACKGROUND
  Cerebral microdialysis (MD) is used to monitor local brain chemistry of patients with traumatic brain injury (TBI). Despite an extensive literature on cerebral MD in the clinical setting, it remains unclear how individual levels of real-time MD data are to be interpreted. Intracranial pressure (ICP) and cerebral perfusion pressure (CPP) are important continuous brain monitors in neurointensive care. They are used as surrogate monitors of cerebral blood flow and have an established relation to outcome. The purpose of this study was to investigate the relations between MD parameters and ICP and/or CPP in patients with TBI.
  
  
  METHODS
  Cerebral MD, ICP and CPP were monitored in 90 patients with TBI. Data were extensively analyzed, using over 7,350 samples of complete (hourly) MD data sets (glucose, lactate, pyruvate and glycerol) to seek representations of ICP, CPP and MD that were best correlated. MD catheter positions were located on computed tomography scans as pericontusional or nonpericontusional. MD markers were analyzed for correlations to ICP and CPP using time series regression analysis, mixed effects models and nonlinear (artificial neural networks) computer-based pattern recognition methods.
  
  
  RESULTS
  Despite much data indicating highly perturbed metabolism, MD shows weak correlations to ICP and CPP. In contrast, the autocorrelation of MD is high for all markers, even at up to 30 future hours. Consequently, subject identity alone explains 52% to 75% of MD marker variance. This indicates that the dominant metabolic processes monitored with MD are long-term, spanning days or longer. In comparison, short-term (differenced or Δ) changes of MD vs. CPP are significantly correlated in pericontusional locations, but with less than 1% explained variance. Moreover, CPP and ICP were significantly related to outcome based on Glasgow Outcome Scale scores, while no significant relations were found between outcome and MD.
  
  
  CONCLUSIONS
  The multitude of highly perturbed local chemistry seen with MD in patients with TBI predominately represents long-term metabolic patterns and is weakly correlated to ICP and CPP. This suggests that disturbances other than pressure and/or flow have a dominant influence on MD levels in patients with TBI.
authors.affiliation: Neurointensive Care Unit, Karolinska University Hospital, Stockholm, Sweden. david.nelson@karolinska.se
authors.forename: David W
authors.initials: DW
authors.lastname: Nelson
authors.forename: Björn
authors.initials: B
authors.lastname: Thornquist
authors.forename: Robert M
authors.initials: RM
authors.lastname: MacCallum
authors.forename: Harriet
authors.initials: H
authors.lastname: Nyström
authors.forename: Anders
authors.initials: A
authors.lastname: Holst
authors.forename: Anders
authors.initials: A
authors.lastname: Rudehill
authors.forename: Michael
authors.initials: M
authors.lastname: Wanecek
authors.forename: Bo-Michael
authors.initials: BM
authors.lastname: Bellander
authors.forename: Eddie
authors.initials: E
authors.lastname: Weitzberg
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D001921:Brain / Q000503:physiopathology*; D001923:Brain Chemistry; D001930:Brain Injuries / Q000175:diagnosis*; D002404:Catheterization / Q000379:methods; D003422:Critical Care / Q000379:methods*; D006801:Humans; D007427:Intracranial Pressure; D017551:Microdialysis / Q000379:methods*; D008875:Middle Aged; D010477:Perfusion; D055815:Young Adult